One
of the real strengths of the USB Digital I/O Commander ("USB Digio") is
its ability to communicate with serial peripheral interface (SPI) devices.
SPI is master/slave protocol utilizing the following four signals:
MISO (master-in-slave-out), MOSI (master-out-slave-in), SCLK (slave-clock),
and SS (slave-select).

MOSI
provides data out from the master, while MISO receives data in to the master.
SCLK provides the slave clock signal for transferring data. And the
SS signal selects the slave device for communication. SS is especially
important for preventing contention when there are multiple slaves to one
master. It ensures that only the desired device is active, while
the outputs of others remain in a high-impedance state. This simple scheme
allows a master (USB Digio) to communicate with many slaves. In fact,
USB Digio software permits you to define up to 10 slaves per master.
And, with up to 10 USB Digio masters hubbed to your PC at once, you can
have a total of 100 slave devices.

So
what kind of slaves can be used with SPI? There are a myriad of devices
including EEPROMs, serial shift registers, real-time clocks, and data converters,
just to name a few. The diagram below gives some additoinal examples:

One master, many
slaves

The
sample schematic below illustrates how to connect two devices (serial-in-parallel-out
shift register and temperature sensor). Digital lines PA0 and PA1
of the master act as the slave-slects.

This
simple circuit has some interesting possibilities. For instance,
create an alarm device that activates a buzzer or LED when the temperature
exceeds a pre-determined threshold. USB Digio can even convert from
raw units to degrees F or degrees C and display on your PC.

(Use
USB Digio as the master controller for this circuit or your own custom
designs. No microcontroller, firmware, or coding skills necessary.
Great for rapid prototyping and proof-of-concept.)